This invention relates to a method capable of accurately ascertaining the state inside a melting furnace employed for immobilizing a radioactive waste in glass, glass ceramics, synthetic rocks and stones, and so forth.
It has been attempted to immobilize and solidify radioactive waste in glass or the like in order to safely and economically store or treat nuclear fission products and radioactive nuclides such as transuranium elements that are contained in the radioactive waste. Research and development in this respect has therefore been made in various countries of the world. When the radioactive waste is immobilized and solidified in the glass, it is first charged into a melting furnace in the liquid state (including a slurry) or in the solid state, is then melted together with the glass and is thereafter withdrawn into a separate container. This method is customary in the art. In order to accomplish a safe and stable operation of the melting furnace in this instance, it is necessary to accurately measure quantities of contents (such as molten glass, unmolten liquid, slurry and its dried and calcined matter, other solid matter, etc.) inside the furnace, their positions inside the furnace and changes in their quantities and positions. Requirements imposed on measuring instruments for this purpose are that they are easy to carry out remote operation, trouble-free and small in size and have high reliabilty because one cannot directly operate or maintain these instruments due to high radioactive intensity around the melting furnace.
In the glass industry, there have conventionally been employed various devices for measuring the level of glass such as, for example, ultrasonic level meters, radiation level meters, electrostatic capacitance type level meters, level meters using thermo-couples, electrode type level meters, and the like. However, these level meters cannot easily be applied to the glass melting furnace used with radioactive waste because their operability, reliability, service life, maintenance, and the like are adversely affected.
Especially when the raw material in the liquid or slurry form is charged into the furnace and is melted with glass, some may be dried and calcined on the surface of the molten matter to thereby form hard solid matter that is, when the raw material is charged into the furnace faster than it can be melted by the molten glass. The solid matter thus formed covers in turn the surface of the molten matter and produces a hard bridge. In such a state, gases, which are primarily generated when the solid matters is melted, accumulate in a space between the molten glass and the bridge of the solid matter, thus elevating the pressure in the space and resulting in furnace damage. Moreover, if the bridge is broken, the gases blow strongly into the furnace and upset the pressure balance. In the case of an over-flow type melting furnace, the glass in the melting furnace is pushed down while the glass at a fore hearth portion is pushed up, thereby also upsetting the pressure balance. If the molten glass alone is withdrawn from the furnace under such conditions where the solid matter forms a bridge, only the level of the glass lowers while the bridge of the solid matter remains as such so that a large cavity is produced under the bridge. In this state, heat conduction from the molten glass diminishes so that the speed at which the solid matters melts decreases and a fall down of the bridge of the solid matter is likely to occur. Accordingly, it is of the utmost importance for the safe operation of the melting furnace to ascertain whether or not the solid matter forms the bridge inside the melting furnace. A conventional method available heretofore for this purpose is to look into and examine the furnace from an inspection window of the furnace. For this reason, development of a suitable detection method has been desired.